Foraminous abrasive articles have been made, for example, as nonwoven abrasive articles constituted of a network of synthetic fibers or filaments which provide surfaces upon which abrasive particles are adhesively attached.
Nonwoven abrasive articles, in particular, are useful in various converted forms, such as wheels, sheets, discs, flap brushes, and the like. In these converted forms, the resulting nonwoven abrasive articles are useful to clean, condition, and/or decorate the surfaces such materials as metal, wood, plastics, glass, ceramics, composites, and the like. A particularly important use for such nonwoven abrasive articles is to scuff automotive body finishes prior to the application of further coatings.
Conventional nonwoven abrasive articles generally involve a mat of fibers which have on at least a portion of their surface an abrasive coating comprising abrasive particles and a binder. As known, the fibers can be formed from various synthetic polymers, including polyamides, polyesters, polypropylene, polyethylene, and various copolymers thereof. Also, naturally occurring fibers such as cotton, wool, bast fibers, and various animal hairs may also be suitable. Suitable abrasive particles can be formed of flint, garnet, aluminum oxide, diamond, silicon carbide, and the like. Binders commonly comprise cured versions of hide glue or varnish, or one or more resins such as phenolic, urea-formaldehyde, melamine-formaldehyde, urethane, epoxy, and acrylic resins. Phenolic resins include those of the phenol-aldehyde type.
In one conventional general scheme, nonwoven abrasive articles have been manufactured by applying to a nonwoven web starting material a "prebond" coating of binder precursor solution, which includes one or more of the above-named resins, in order to impart sufficient strength to the nonwoven web starting material so that it can better tolerate subsequent processing. A "make" coating optionally has been applied to the prebonded nonwoven web and left non-fully cured up until the time when abrasive particles are later applied to the web to help attach the abrasive grains throughout the lofty fibrous mat. Finally, an abrasive coating of resinous binder material and abrasive particles has been applied onto the nonwoven to increase the abrasive characteristics of the nonwoven. The resin binder used in the various coatings may be the same or different.
In a conventional approach, the binder coating for the abrasive particles has been applied to the nonwoven web as a non-foamed dispersion of a binder resin and abrasive particles in a liquid medium. The dispersion is then applied to the nonwoven substrate by means, such as spraying, that either atomizes the mixture to atomized droplets, or otherwise causes a film of the dispersion to be formed on the web. The atomized droplets or film are applied to the nonwoven web and cured. During curing, the droplets either flow together (coalesce) by heat-induced viscosity reduction, or cure as individual droplets where they stand. Films usually flow together to cover (wet) most of the filaments in the nonwoven web, and they are cured in position.
As the binder resin, phenolic resins are used extensively to manufacture nonwoven abrasive articles because of their thermal properties, availability, low cost, and ease of handling. The monomers currently used in greatest volume to produce phenolic resins are phenol and formaldehyde. Other important phenolic starting materials are the alkyl-substituted phenols, including cresols, xylenols, p-tert-butylphenol, p-phenylphenol, and nonylphenol. Diphenols, e.g., resorcinol (1,3-benzenediol) and bisphenol-A (bis-A or 2,2-bis(4-hydroxyphenyl)propane), are employed in smaller quantities for applications requiring special properties.
There are two basic types of phenolic resins: resole and novolak phenolic resins. Molecular weight advancement and curing of resole phenolic resins are catalyzed by alkaline catalysts. The molar ratio of aldehyde to phenolic is greater than or equal to 1.0, typically between 1.0 and 3.0. In the production of adhesive coatings for nonwoven abrasives, one standard starting phenolic resin composition is a 70% solids condensate of a 1.96:1.0 formaldehyde:phenol mixture with 2% potassium hydroxide catalyst added based on the weight of phenol. The phenolic resin composition is typically 25-28% water and 3-5% propylene glycol ether, which are required to reduce the viscosity of the resin. In conventional techniques for making nonwoven abrasives, the phenolic resin has not been applied to the nonwoven web substrate in foamed condition.
It is possible for nonwoven abrasive articles resulting from these conventional coating methods to have an abrasive particle distribution through the thickness of the web such that abrasive particles somewhat more concentrated near the external surface regions of the mat (due to spray coating). However, substantial quantities of abrasive particles nonetheless are usually present throughout the web including the interior regions. The abrasive particles lodged in the central interior regions of the web are not immediately available for useful abrading work until the external surface(s) of the article is worn or otherwise attrited.
For applications in automotive body finishing, the ability of nonwoven abrasive articles to quickly create a uniformly scratched surface is of primary consideration. For this purpose, it would be advantageous to have the abrasive particles concentrated at the external surfaces of such nonwoven articles thereby increasing the number of such particles in immediate, simultaneous contact with the workpiece, instead of being more or less uniformly distributed throughout the thickness of the web.
The state of the art can be further understood by reference to the following references in particular:
U.S. Pat. No. 2,958,593 (Hoover et al.) describes a low density, open, nonwoven fibrous abrasive article formed of fibers formed into a nonwoven web, abrasive particles, and a curable binder. Organic fibers are adhesively bonded together at crossing and contacting points, and abrasive particles are also adhesively bonded to the web fibers. The interstices between the fibers are left open and unfilled by adhesive or abrasive particles so that the web is non-clogging and non-filling in nature, and it consequently can be readily cleaned upon flushing. The adhesive used to bond the contacting points of the fibers in the web can also be used as the means of attaching the abrasive particles to fibers in the web. The abrasive mineral particles are sprayed onto the nonwoven web as dispersed in the liquid binder solution. Alternatively, the mineral binder can be roll coated, dip coated, or separately applied relative to the abrasive particles, upon the nonwoven web. For instance, the mineral binder can be first sprayed upon the web followed by sifting of the abrasive mineral particulate upon the resin wetted web.
U.S. Pat. No. 3,175,331 (Klein) discloses a cleaning and scouring pad comprising one or more fibrous batts, heat-sealed so as to be capable of having enclosed therein a solid washing composition, and in which the outer surface of the pad has grit adhered thereto to provide a continuous, uninterrupted scouring surface extending over the entire outer surface of the pad. A fusible adhesive in liquid form is applied onto either or both surfaces of the fibrous batt sufficient to lightly impregnate the outer surface only of the batt to bond outer fibers together while not filling voids between fibers or penetrating to fibers on the opposite surface of the batt. Therefore, the amount of adhesive is regulated in order to concentrate the adhesive in the area of the surface of the batt instead of the interior of the batt. Where scouring action is desired, abrasive grit is pre-mixed into the impregnating adhesive and applied to the surface(s) of the fibrous batt, such as by spraying. Where the finished pad is to be used for washing instead of scouring, the abrasive material can be omitted from the impregnating resin used in Klein.
However, in order to limit the penetration depth of the liquid resin into the batt as in Klein, careful and time-consuming pre-consideration and monitoring during processing of many parameters, such as resin coating amount, resin flowability, resin viscosity, web thickness, web density, and so forth, would be required.
French Patent Application Publication No. 2,409,095 discloses the use of collapsible, colored foams to concentrate pigments near a surface of a porous support, such as a fibrous support. The pigments or ink base materials are incorporated into and intimately admixed with a binder in dispersion with a mechanical foam machine. A colored foam is formed in which the pigment and binder resins are temporarily suspended within a foam. The resulting colored foam is applied to a fabric surface and subjected to heat to collapse the foam such that the pigment and binder residues only penetrate the fabric surface to a limited degree. French patent 2,409,095 thus allows for surface printing or dyeing of a side or both sides of a fabric without strike-through problems. French patent 2,409,095 teaches pigments, usually relatively light and soft materials, and not dense, solid granulate, as suspended, along with the binder, within and throughout the bulk of the foam. Therefore, pigments located at the lower regions of the foam layer will contact the fabric surface immediately after, or very shortly after, the foam is applied to the fabric surface.
U.S. Pat. No. 4,969,975 (Biggs et al.) describes a process by which a homogeneous sheet comprising a uniform distribution of fibers and/or particles, which otherwise might float and/or settle, by incorporating the fibers and/or particles into a froth or foam itself, and depositing and draining the foamed dispersion on a fibrous support. As the particle additives are distributed within and throughout the foam in Biggs et al., at least a portion of such additives will immediately come into contact with the substrate used as the coating support.
As can be appreciated from the above, there still remains a need for a technique to concentrate abrasive particles in the external surface regions of a foraminous abrasive article, such as a nonwoven, by a simple manufacturing scheme that does not require extensive preparation and monitoring during processing.